Diabetes, including type 1 (T1D) and type 2 (T2D) diabetes, is a major public health problem, costing over $100 billion annually in related health care. Diabetes eventually results from an inadequate number of functional beta cells. Regeneration or proliferation of human beta cells is extremely slow and inefficient in diabetic conditions, presenting a great hurdle to regenerate beta cells for ameliorating diabetes. In this regard, mutations in the multiple endocrine neoplasia type 1 gene (MEN1), which encodes the nuclear protein menin, is the only genetically proven means to effectively increase proliferation of beta cells in humans. Menin is physiologically inhibited to increase beta cell proliferation to prevent gestational diabetes. Our recent findings demonstrate that acute Men1 excision reverses pre-existing hyperglycemia in mice fed with high-fat diet (HFD). However, it is not well understood how inhibition of menin leads to increased beta cell regeneration. Recently, we helped solve the co-crystal structure of menin and JunD, and found that menin harbors a deep pocket for binding to JunD and inhibits JunD phosphorylation. Moreover, both menin and JunD bind to the promoter of the endogenous cyclin D1 gene, a crucial proliferation factor in beta cells. Furthermore, menin was found to interact with a histone arginine methyltransferase, leading to suppression of expression of other pro-proliferative genes and Hedgehog (Hh) signaling, and a pro-proliferative pathway. Thus, it is plausible to hypothesize that menin normally suppresses expression of cyclin D1 via repressing JunD, and also represses other proliferative genes and Hh signaling, in concert with histone arginine methyltransferase, to suppress beta cell regeneration. To test these hypotheses, three aims are proposed: Aim 1. Investigate how menin controls expression of cyclin D1 via regulating JunD. Aim 2. Examine the role of the histone arginine methyltransferase in controlling gene expression, beta cell regeneration, and glucose tolerance in mouse models. Aim 3. Investigate menin-mediated regulation of Hh signaling in controlling beta cell regeneration. These studies will likely unravel novel mechanisms of beta cell regeneration, paving the way to develop a novel menin pathway-based therapy to treat diabetes.